In recent years, research of electronic devices such as organic thin film transistors and the like receive attention, and various materials to be used for the electronic devices are investigated. Among these, development of materials for forming an insulating layer included in the electronic device is actively carried out in order to suppress deterioration of the electronic device over time.
Since an organic thin film transistor which is one aspect of the electronic device can be produced at lower temperatures than inorganic semiconductors, a plastic substrate or film can be used as a substrate of the organic thin film transistor, and by using such a substrate, a device which is flexible, and is lightweight and is hardly breakable can be obtained. Moreover, since there are cases where a device can be produced by film formation using a method of applying or printing a solution containing an organic material, there are also cases where a large number of devices can be produced on a substrate of large area at low cost.
Furthermore, since there are a wide variety of materials which can be used for the investigation of transistors, a device with a wide range of varied characteristics can be produced by using materials varying in molecular structure in the investigation.
Further, organic semiconductor compounds to be used for an electric field effect type organic thin film transistor which is one aspect of organic thin film transistors are susceptible to environmental influences, such as humidity and oxygen, and therefore transistor characteristics are likely to be deteriorated over time due to humidity, oxygen, etc.
Therefore, in the device architecture of a bottom-gate type organic thin film transistor, which is one kind of electric field effect type organic thin film transistors, with an organic semiconductor compound exposed thereon, it is necessary to form an overcoat insulating layer covering the whole structure of the device so as to protect the organic semiconductor compound from being in contact with the open air. On the other hand, in the device architecture of a top gate type organic thin film transistor, which is another kind of electric field effect type organic thin film transistors, an organic semiconductor compound is coated and protected with a gate insulating layer. Thus, in organic thin film transistors, insulating layer materials are used in order to form an overcoat insulating layer and a gate insulating layer, both of which cover the organic semiconductor layer.
The organic thin film transistor insulating layer material is required to have electrical insulating properties and characteristics superior in electrical breakdown strength when having been formed into a thin film. Further, particularly in the bottom-gate type electric field effect type organic thin film transistor, an organic semiconductor layer is formed on the gate insulating layer. Therefore, the organic thin film transistor gate insulating layer material is required to have affinity with an organic semiconductor compound for forming an interface in close contact with the organic semiconductor layer and to have flatness of the surface on the organic semiconductor layer side of the film formed from the organic thin film transistor gate insulating layer material.
As a technology responding to such requirements, Patent Document 1 describes that an epoxy resin and a silane coupling agent are used in combination as an organic thin film transistor gate insulating layer material. In this technology, a composition containing an epoxy resin and a silane coupling agent is heated to 150° C., and thereby, a hydroxyl group produced at the time of a curing reaction of the epoxy resin is reacted with the silane coupling agent. The reason for this is that the above-mentioned hydroxyl group enhances the hygroscopic properties of the gate insulating layer material and impairs the stability of transistor performance.
Non-Patent Document 1 describes the use of a resin prepared by thermally cross-linking polyvinylphenol and a melamine compound at 175° C. for a gate insulating layer. In this technology, by cross-linking with the melamine compound, the hydroxyl groups contained in the polyvinylphenol are removed and the film strength is increased simultaneously. A pentacene TFT having this gate insulating layer has low hysteresis and exhibits durability to a gate bias stress.
Non-Patent Document 2 describes that polyvinylphenol and a copolymer prepared by copolymerizing vinylphenol with methyl methacrylate are heated at 150° C. and used for a gate insulating layer. In this technology, the polarity of the whole film is reduced by interactions between the hydroxyl group of vinylphenol and the carbonyl group of methyl methacrylate. A pentacene TFT having this gate insulating layer has low hysteresis and exhibits stable electric properties.